1. Field of the Invention
The subject invention relates to printed circuit cards for add in functions for computer systems. More particularly, the invention relates to a method and apparatus for adding bandwidth to such cards through additional bus type connectors without changing the form factor.
2. Description of the Prior Art
The PCMCIA (Personal Computer Memory Card International Association) standard was developed for the purpose of providing user installed memory and I/O functions for small form factor digital computer systems. The standard specifies a card containing a printed circuit board. This product is usually referred to as a a PC card. There are three PC card formats: Types I, II and III. All three have external dimensions of 54 millimeters by 85.6 millimeters. Thicknesses vary. Type I is 3.3 millimeters thick. Type II is 5 millimeters thick and Type III is 10.5 millimeters thick. The cards look similar to a conventional credit card. The standard specifies a 68 position connector on one end. The 68 position connector plugs into a mating connector mounted on a header which is in turn mounted to a mother board or daughter board which is located inside the host. The header is U shaped with the 68 pins at the base of the U. There is a wide variation of headers including headers for different thickness cards; however, the 68 pin connector is common to all PCMCIA cards.
The original PC cards were for memory addition and thus had no interaction with external devices. I/O cards were developed later to add functions such as modems, faxes, network interfaces, multi-media interface, sound cards, etc. In order to handle I/O functions, a second connector is located on the end of the card opposite the 68 pin connector. The I/O connector reaches the outside world through a cable.
The PCMCIA standard specifies the function of each of the 68 pins in the connector. The standard supports either an 8 bit or 16 bit bus. There are four ground pins, two power pins and up to 3 free signal pins for additional functions. With only four ground pins, ground is not distributed. This effectively limits the bandwidth to about 20 mhz. With only 2 power pins, the power function is even less distributed. Indeed, the power is limited to 3.3 watts. The number of pins in the connector effectively limits the bandwidth of the system. But computer hardware continues to move ahead in performance. Thus, if the PCMCIA cards are to keep pace, a way must be found to improve their performance while retaining backwards compatibility with existing cards.
This can be done by only adding additional connector pins to the standard in such a way that backward compatibility is not sacrificed. This means for example that the physical size cannot change and the 68 pin connector must be retained and in precisely the same location that it now commands.
Sideswipe connector systems as described in the co-pending application referenced in the first section and assigned to the same Assignee as the present invention would provide additional contacts to the card. But this approach is limited to 10 pins. Also, for bus applications, the contacts must be in close proximity to the address, control and data signals. There is a specific need for more ground pins with higher bandwidth signals to deal with the added noise in higher frequency systems. And the PCMCIA standard in addition to limiting the total number of pins, also limits the number of ground pins. Ideally, the grounds must be very close physically to the data lines. The reason for requiring close proximity is to minimize the generation of common mode noise which can occur if the distance that electrical signals have to travel from input pin to ground is not kept to a minimum. Typically in high performance systems ground connections are distributed. But in the PCMCIA standard, this was not done since it was not intended for high speed operation.
An alternate concept that has been considered adds a grounding shield to the top of the 68 pin socket on the card. This solves distributed grounding across the card and is backward compatible. But, this approach requires a cutout in the top cover of the card where the connection to the 68 position connector occurs. As a result this approach does not have the mechanical strength to handle repeated insertions and removals. In addition, this approach does not solve the problem of providing additional power and signal pins. Finally, inserting a current PCMCIA card into such a socket may cause the grounding shield pins to be fouled by the paint on the top surface of such cards.